Charged-particle-beam lithography systems have been developed for transferring patterns from masks to sensitized substrates. Such systems permit the rapid transfer of-high-resolution patterns to the sensitized substrate and are important in the manufacture of electronic devices, particularly integrated circuits.
Conventional charged-particle-beam lithography systems are static transfer systems that transfer the pattern of a single die (a die is equivalent to one integrated circuit) or of multiple dies in a single transfer (exposure) step. These conventional systems face several difficulties. First, the pattern mask for a static transfer system is difficult to fabricate. Second, the control of aberrations in the charged-particle-beam optical system (termed generally the "optical system" herein) across an optical field as large as one or more dies requires complex optical elements.
To overcome the difficulties summarized above, charged-particle-beam lithography systems have been developed in which the pattern on the mask is divided into multiple smaller regions termed "subfields." Such systems transfer the pattern on the mask subfield-by-subfield. In such a "subfield-by-subfield" lithography system, the mask is divided into a plurality of smaller regions. The patterns in the smaller regions are then transferred one at a time to the sensitized substrate. The transfer of the patterns in these smaller regions is less demanding on the optical system so that optical aberrations are reduced and high-resolution patterns are more readily transferred to the sensitized substrate. Because the mask is divided into smaller regions, mask fabrication is simpler.
Subfield-by-subfield pattern-transfer systems are commonly used in the manufacture of integrated circuits. With such systems, the sensitized substrate is generally a semiconductor wafer that is coated with a thin layer of resist, the "resist" being a material sensitive to the charged particle beam. In the subfield-by-subfield transfer method, the mask and the wafer are moved continuously and synchronously together in a fixed direction (hereinafter "scan direction") by their respective mounting stages. The mask pattern to be transferred is divided into a series of field bands extending in a direction perpendicular to the scan direction. During scanning, as each field band approaches the optical axis of the optical system, the patterns within the field band are transferred to the wafer. By exposing regions of the wafer near the optical axis to the charged particle beam, optical aberrations are reduced.
The height of the wafer surface generally varies across the wafer surface. Not only are there variations in wafer thickness but also there may be particles between the wafer and its mounting stage, causing the wafer to deform when held in the stage. If these height variations are not corrected, the surface of the wafer being exposed by the charged particle beam may be beyond the depth of focus of the optical system, causing blurring in the image of the projected patterns. The wafer may also be tilted with respect to the optical system; this tilt may appear as an optical-system defocus that varies across the wafer surface.
According to one conventional method for overcoming wafer-height variations, deformations, and tilt, the height of the wafer is measured at a measurement point on or near the region in which the wafer is being exposed to the charged particle beam. The result of this measurement is then used either to electrically correct the focus of the optical system or to move the wafer with a mechanical height adjustment to the wafer mounting stage.
In conventional systems, the wafer height is measured at a point on the wafer within or near the exposure region of the lithographic system. Because the wafer is moving continuously in the scan direction, the measurement point moves before the focus corrections are made. If T.sub.A and T.sub.B are the times required to measure the wafer height and subsequently correct the optical system focus respectively, and if the scanning speed of the wafer is V.sub.W, the wafer will move a distance (T.sub.A +T.sub.B).multidot.V.sub.W in the time required to measure and correct focus errors. As a result, conventional systems are generally unable to maintain focus if there are rapid changes in wafer height and large focus errors are possible. This causes a loss of resolution in the transferred patterns in wafer regions with rapid changes in wafer height.